1. Field of the Invention
The present invention relates to a precursor sol of aluminum oxide, the precursor sol of aluminum oxide having satisfactory coating properties and being capable of providing satisfactory antireflection performance in a wide range including a visible region when the precursor sol of aluminum oxide is subjected to baking treatment even at a low temperature, and relates to an optical member and a method for producing an optical member using the precursor sol of aluminum oxide.
2. Description of the Related Art
An antireflective structure with a microstructure which is equal to or smaller than a wavelength in the visible region and which has an appropriate pitch and height is known to provide excellent antireflection performance in a wide wavelength range. It is known that a method for producing the microstructure includes, for example, forming a film by application in such a manner that fine particles each having a particle size of the wavelength or less are dispersed in the film.
It is known that the employment of a method for producing a microstructure by patterning with a microprocessing apparatus, for example, an electron beam lithography apparatus, a laser interference exposure apparatus, a semiconductor exposure apparatus, or an etching apparatus, enables us to control the pitch and height of the microstructure, resulting in a microstructure having excellent antireflection performance (for example, see Japanese Patent Laid-Open No. 50-70040).
An example of other known methods is a method in which boehmite, which is an aluminum hydroxide oxide, is grown on a base to provide antireflection performance. In these methods, an aluminum oxide (alumina) film formed by a vacuum film-formation method or a liquid-phase method (sol-gel method) is subjected to steam treatment or hot-water immersion treatment to turn the surface layer of the film into boehmite, thereby forming an antireflection coating film (for example, see Japanese Patent Laid-Open No. 9-202649).
It is known that in the method for forming an antireflection coating film with a microstructure composed of boehmite, the resulting film has extremely low normal-incidence reflectivity and oblique-incidence reflectivity and provides excellent antireflection performance.
However, in the case where a sol in which an organic aluminum compound is easily aggregated is applied on a member to form a film, the film can have a low degree of uniformity, leading to poor appearance due to the nonuniformity of the film.
In the case of employing a wet process, for example, a liquid-phase method (sol-gel method), the baking temperature during film formation is usually as high as 200° C. or higher (for example, see Japanese Patent Laid-Open No. 9-202649). This causes problems in which peripheral members and the figure tolerance of an optical member are adversely affected and in which a film cannot be formed on a base, such as a resin base, which does not withstand high temperatures. A sol configured to form an aluminum oxide film by a liquid-phase method (sol-gel method) contains a stabilizer to prevent clouding of the film due to rapid hydrolysis of an aluminum alkoxide by water in air or the addition of water. Typical examples of the stabilizer that can be used include β-ketoester compounds, β-diketone compounds, and alkanolamines.
The stabilizer can be chelated with aluminum to form an organic aluminum compound, causing aggregation. The resulting organic aluminum compound can have a high melting point or sublimation point. So, if baking is performed at a temperature lower than 200° C., the organic aluminum compound remains in the resulting aluminum oxide film. Furthermore, it is speculated that the aggregation of the organic aluminum compound inhibits bonding formation between particles by the hydrolysis of the aluminum alkoxide in an optical film during baking. If the bonding formation between particles is not sufficient, a textured structure composed of aluminum oxide boehmite is not sufficiently formed, thereby degrading the antireflection performance.
In order not to cause poor appearance due to the nonuniformity of the film or inhibition of the bonding formation between particles during film formation, the aggregation of the organic aluminum compound can be inhibited. However, it is difficult to inhibit the aggregation of the organic aluminum compound while the sol is maintained at a stable state without controlling the structure of the organic aluminum compound and thus to provide an optical film having satisfactory appearance and antireflection performance by baking at a temperature of lower than 200° C.
In a liquid-phase method (sol-gel method) to form an antireflection coating film from such a precursor sol of aluminum oxide, there is a demand for a precursor sol of aluminum oxide such that the appearance of an optical member is not impaired by the aggregation of the organic aluminum compound. Furthermore, there is a demand for a precursor sol of aluminum oxide capable of being formed into a high-performance antireflection coating film at a lower temperature, an optical member, and a method for producing an optical member using the precursor sol of aluminum oxide.
The present invention has been made in view of the related art described above. Aspects of the present invention provide an optical member in which the nonuniformity of an antireflection coating film is minimized and in which satisfactory antireflection performance is achieved even when baking treatment is performed at a low temperature, and provide a method for producing the optical member. Furthermore, aspects of the present invention provide a precursor sol of aluminum oxide, the precursor sol inhibiting the aggregation thereof, and a method for producing the precursor sol of aluminum oxide.